Inorganic nanoclusters dispersed in organic matrices are of importance to a number of emerging technologies. However, obtaining a set of desired properties with such organic-inorganic composites may require high concentrations of well-dispersed nanoclusters. Achieving high concentrations is possible when the chemistries of the particle surface and of the matrix are selected to produce a relatively low value of the interfacial free energy; an excess of interfacial free energy may cause phase separation and sometimes aggregation of nanoparticles. Thus, one type of system that achieves these energy characteristics is one in which the nanoclusters are stabilized by molecular species that are components of the encapsulating matrix or encapsulant. Since typical organic matrices are chosen as encapsulants for their bulk properties, they may not exhibit chemistries that provide low interfacial energies with nanoclusters. Also, the organic-inorganic interface can play a significant role in establishing and maintaining the desired nanocluster (and hence composite) properties, placing further constraints on desirable encapsulant/nanocluster combinations.
Ormosil is an acronym that stands for organically modified silicates. This term was initiated by Schmidt in the early 1980's to describe sol-gel materials comprised of an organoalkoxysilane, a Si—O inorganic backbone with pendant organic groups. A desirable feature of ormosils is their utility for creating organic-inorganic hybrid materials being standard sol-gel techniques. Ormosils can be formed from single monomeric precursors or mixtures of monomers. When ormosil compounds are derived from monomers (or mixtures of monomers) such that R′4-xSi(RO)x where x is, on average between 3 and 4, polymerization leads to a 3-dimensional Si—O—Si network. The reactive alkoxy (RO) groups crosslink to form a 3-dimensional network and the final solid material contains properties imparted by the organic R′ moiety. Thus, R′ may be chosen to enhance chemical compatibility between particular nanoclusters and the reactive polymer matrix without interfering with the network forming R-groups, which largely determine the optical quality of the matrix. Other advantages of this family of materials are high condensation efficiencies and resulting low porosity.
Seddon and Ou (A. B. Seddon and D. L. Ou, “CdSe Quantum Dot Doped Amine-Functionalized Ormosils,” J. Sol-Gel Sci. Technol. 13 (1998) p. 623-628) have isolated nanosized phenyl-capped CdSe particles (quantum dots) after preparation inside reversed micelles present in AOT/H2O/heptane and redispersed them in amine-functionalized ormosils derived from 3-aminopropyl(trimethoxy)silane.
Mulvaney and Liz-Marzan (P. C. Mulvaney and L. M. Liz-Marzan, U.S. Pat. No. 6,548,168) claim a method for preparing a coated particle comprising admixing a source of a particle, a source of coating, and a bifunctional ligand and allowing the bifunctional ligand and coating to deposit on the particle.
Silicones are macromolecules comprised of a polymer backbone of Si and O with organic side groups attached to the Si atoms. Silicones are also termed polysiloxanes. Selection of suitable organic side groups can enable cross-linking to form a polymer network.
Kambe and coworkers (Kambe et al., U.S. Pat. No. 6,881,490) report inorganic particle/polymer composites that involve chemical bonding between the elements of the compound. The inorganic particles are formed prior to their reaction with the polymer to form the composite. In a first aspect, the invention of Kambe et al. pertains to a composite composition comprising a polymer having side groups, chemically-bonded to inorganic particles. In another aspect, the invention pertains to a composite composition comprising inorganic particles chemically bonded to a polymer through a linkage comprising a plurality of functional groups, the polymer selected from a group including polysiloxanes. The polymers posses functional side groups and/or terminal sites that can be chemically bonded with the inorganic particles, which generally are functionalized by bonding with a linker compound. In a further aspect, the invention of Kambe et al. pertains to a composite composition comprising a polymer chemically bonded to inorganic particles, wherein the inorganic particles comprise a metal. In addition, the invention pertains to a collection of metal/metalloid oxide or metal/metalloid nitride particles that are chemically bonded through a chemical linkage comprising an amine group, an amide group, a sulfide group, a disulfide group, an alkoxy group, a ester group, an acid anhydride group. The linkage is chemically bonded with a polymer. In other aspects, the invention pertains to a method for forming chemically bonded polymer inorganic particle composites. The method comprises binding side chain functional groups of polymer units to functional groups of a linker compound bonded to the inorganic particles.
In some embodiments of Kambe et al., the polymer incorporates the inorganic particles into the polymer network. This can be performed by reacting a functional group of the linker compound with terminal groups of a polymer molecule. Alternatively, the inorganic particles can be present during the polymerization process such that the functionalized inorganic particles are directly incorporated into the polymer structure as it is formed. In other embodiments, the inorganic particles are grafted onto the polymer by reacting the linker functional groups with functional groups on polymer side groups. The inorganic particles generally include metal or metalloid elements in their elemental form or in compounds. Specifically, the inorganic particles can include, for example, elemental metal or elemental metalloid, i.e. un-ionized elements, metal/metalloid oxides, metal/metalloid nitrides, metal/metalloid carbides, metal/metalloid sulfides or combinations thereof. Metalloids are elements that exhibit chemical properties intermediate between or inclusive of metals and nonmetals. Metalloid elements include silicon, boron, arsenic, antimony, and tellurium. Preferred particles have an average diameter of less than about 500 nanometers (nm). Suitable nanoparticles can be formed, for example, by flame synthesis, combustion, or sol gel approach. Preferred methods for synthesizing the particles include laser pyrolysis in which light from an intense focused source drives the reaction to form the particles. Laser pyrolysis is useful in the formation of particles that are highly uniform in composition, crystallinity and size. To form the desired composites, the inorganic particles are modified on their surface by chemical bonding to one or more linker molecules. The ratio of linker composition to inorganic particles preferably is at least one linker molecular per inorganic particle. The linker molecules surface modify the inorganic particles, i.e., functionalize the inorganic particles. While the linker molecules bond to the inorganic particles, they are not necessarily bonded to the inorganic particles prior to bonding to the polymers. They can be bonded first to the polymers and only then bonded to the particles. Alternatively, they can bond to the two species simultaneously.